1. Field of the Invention
The present invention relates to a method for forming a thin film on a substrate. More particularly, the present invention relates to a method for forming a thin film that is suitable for forming a transparent conductive film used in an image display apparatus such as a liquid crystal display.
2. Description of the Prior Art
Generally, a pixel electrode substrate used in a liquid crystal panel is produced by the following method. A transparent conductive substrate is first produced by forming a thin film made of indium tin oxide (hereinafter, referred to as xe2x80x9cITOxe2x80x9d) by reactive sputtering as a transparent conductive film on the entire surface of a transparent substrate. Then, pixel electrode patterning is performed with respect to the ITO thin film by etching so as to produce a pixel electrode substrate.
The reactive sputtering for the ITO thin film used in this method is performed by introducing inert gas and reactive gas into a sputtering apparatus. A sputtering target for discharge power application, a substrate holder for holding a substrate to be treated, a shutter located between the sputtering target and the substrate holder are provided in the sputtering apparatus.
Prior to the formation of a thin film by reactive sputtering, sputtering discharge is performed with gas introduction and discharge power application while the shutter is closed before a substrate to be treated is introduced into the sputtering chamber (hereinafter, the sputtering discharge without a substrate to be treated being introduced in the sputtering chamber is referred to as xe2x80x9cidling dischargexe2x80x9d), in order to stabilize a plasma discharge state in sputtering discharge.
Conventionally, it has been common that this idling discharge is performed for a short time such as 3 to 5 minutes and at most about 10 minutes.
Furthermore, the discharge power during the formation of a thin film is set to a low power value in order to stabilize reactive sputtering. On the other hand, the discharge power during the idling discharge is set to a power higher than the discharge power applied for the formation of a thin film in order to speed up the cleaning of the surface of the target for shortening the treatment time and to reduce variations in the plasma discharge state caused by opening and closing the shutter.
Conventionally, a black and white liquid crystal panel has been displayed by determining at each pixel whether the pixel is bright or dark, i.e., whether or not light is transmitted. For this reason, the quality of a transparent conductive film is determined by whether or not light is transmitted, and does not significantly depend on the transmittance of the transparent conductive film.
However, since a color liquid crystal panel is displayed with three primary colors of red, green and blue, a non-uniform transmittance in the transparent conductive substrate results in unintended reproduction of colors in the liquid crystal panel. Furthermore, when transparent conductive substrates have significantly different transmittances from each other, liquid crystal panels that have been produced with the transparent conductive substrates reproduce colors differently. Therefore, in the production of color liquid crystal panels, when transparent conductive films are formed on a large number of substrates, it is important for each of the transparent conductive substrates to have a uniform transmittance within its own substrate. Moreover, it is important for all the transparent conductive substrates to have a uniform transmittance.
Generally, the sheet resistance and the transmittance of a transparent conductive film formed of a metal oxide are correlated closely. Stable sheet resistance and low levels of dispersion in the sheet resistance result in stable transmittance. However, as a result of study in detail by the inventors as to conventional methods for forming a thin film such as a transparent conductive film, the inventors found a first problem in that the dispersion in the sheet resistance of transparent conductive films successively formed on a plurality of substrates by reactive sputtering and the sheet resistance in each of the transparent conductive substrates depends on the order of the formation of the films, namely, the order of the introduction of substrates to be treated to the sputtering chamber.
The first problem in conventional methods for forming a thin film will be described with reference to FIGS. 6, 7, 8 and 9.
FIG. 6 shows a sequence of discharge power applied for formation of ITO thin films on 50 substrates to be treated. First, gas is introduced into a sputtering chamber, a discharge power of 450 W is applied for idling discharge and a first substrate to be treated is transported into a sputtering chamber and positioned on a substrate holder (during a period shown as T1 in FIG. 6). Then, the discharge power is dropped to 400 W and the shutter is opened, and an ITO thin film is formed on the first substrate to be treated so that a first transparent conductive substrate is produced (during a period shown as T2 in FIG. 6). The shutter is closed and the discharge power is raised to 450 W, and the first transparent conductive substrate is transported out from the sputtering chamber before a second substrate to be treated is transported into the sputtering chamber and positioned in the substrate holder (during a period shown as T3 in FIG. 6). Then, the discharge power is dropped to 400 W and the shutter is opened, and an ITO thin film is formed on a second substrate to be treated (during a period shown as T4 in FIG. 6). The same film-formation cycles are repeated until an ITO thin film is formed on a 50th transparent conductive substrate (during a period shown as T100 in FIG. 6). Then, the shutter is closed and the discharge power is raised to 450 W, and the 50th transparent conductive substrate is transported out from the sputtering chamber (during a period shown as T101 in FIG. 6). Then, the discharge power is stopped and thus the process for forming the transparent conductive films is completed.
FIG. 7 shows changes in the sheet resistance of the transparent conductive substrate according to the order of the ITO thin film-forming process by the conventional method for forming a thin film as described above. FIG. 8 shows the dispersion in the sheet resistance in each transparent conductive substrate. As seen from FIGS. 7 and 8, the sheet resistance is high and the dispersion in the sheet resistance is large with respect to the ITO thin films on the first to fifth transparent conductive substrates, i.e., the substrates treated with reactive sputtering in an early stage of the sequential forming process. The dispersion in the sheet resistance represents a difference from an average value of values obtained by measuring a sheet resistance at 5 points on a surface of a transparent conductive substrate with a 150 mm diameter.
The gas state in the sputtering chamber during the formation of ITO thin films by the conventional method shown in FIG. 6 is analyzed with a quadruple mass spectrometer. FIG. 9 shows pressure changes of introduced gases, i.e., argon (Ar), hydrogen (H2) and oxygen (O2), and water (H2O) calculated with mass numbers obtained by the mass analysis.
FIG. 9 shows the results with respect to a period before start of discharge, an idling discharge period 60, and a period until a thin film is formed on a 10th substrate in the period for sequentially forming ITO thin films on the 50 substrates. Sharp falling portions seen in the pressure change result of each gas are caused by the switching of discharge power between 450 W for idling discharge and 400 W for formation of the thin films and the opening and closing of the shutter. The ITO thin films are formed on sequentially introduced substrates during periods segmented by the falling portions.
As seen from FIG. 9, the pressures of Ar, H2 and O2 are substantially constant except for the initial period with gas introduction and the start of idling discharge and the falling portions, whereas the pressure of H2O changes along a general shape of a gentle slope during a period 61 from the initial period until the formation of a thin film on the fifth transparent substrate. H2O is present as a residual gas in the sputtering chamber prior to the gas introduction and the idling discharge. Compared with the pressure of the H2O at that period, the pressure of H2O is higher after the gas introduction and the idling discharge. This means that H2O increases as an intermediate product formed from introduced H2 and O2 by sputtering discharge. Furthermore, the pressure of the H2O formed by the sputtering discharge changes along a general shape of a gentle slope during a period 61 from the start of idling discharge until the formation of a thin film on the fifth transparent substrate.
The fact that the pressure of H2O is stable after the formation of a thin film on the sixth substrate indicates that the pressure of H2O increases because H2O as an intermediate product is produced in large amount after the start of idling discharge, and the pressure of H2O becomes stable as a certain period has passed. Since the other introduced gases are present in a large amount, compared with H2O, the change after the start of idling discharge that is seen in H2O was not detected.
The above-described analysis results together with the reference to FIG. 6 suggest that the changes in the pressures and the ratio of the gas partial pressures since the gas introduction and the start of idling discharge influence the sheet resistance of the transparent conductive films and the dispersion in the sheet resistance in each transparent conductive substrate.
Furthermore, as a result of study in detail of the inventors as to conventional methods for forming a thin film such as a transparent conductive film, the inventors found a second problem as follows when a plurality of substrates are successively subjected to reactive sputtering. In the case where the idling discharge power after the start of reactive sputtering is set higher than the discharge power for formation of ITO thin films, residue appears after etching is performed to form pixel electrode pattern with respect to the transparent conductive substrates that have been produced early in the order of the formation of the thin films.
When the idling discharge power after the start of sputtering discharge is set higher, particles sputtered from a sputtering target reach a substrate in a high energy state because the particles have been exited during the idling discharge. This results in changed quality of the ITO films, thus causing such residue.
Therefore, with the foregoing in mind, it is the object of the present invention to provide a method for forming uniform thin films regardless of the order of the formation of the films.
An embodiment of a method for forming a thin film of the present invention includes the steps of performing idling discharge for reactive sputtering with the introduction of sputtering gas and the application of discharge power in a sputtering chamber until a pressure of a product produced from the sputtering gas by the idling discharge reaches a stationary state; and forming films on substrates in the sputtering chamber by the reactive sputtering while introducing the substrates to be treated into the sputtering chamber and removing the substrates from the sputtering chamber, one at a time, sequentially.
According to this embodiment, the pressure of introduction gas produced after the start of the idling discharge and the pressure of an intermediate product produced by plasma can be stable. This embodiment has an effect, especially on solving the first problem.
Furthermore, another embodiment of a method for forming a thin film includes the steps of performing idling discharge for reactive sputtering with the introduction of sputtering gas and the application of discharge power in a sputtering chamber in such manner that a discharge power at the idling discharge is not more than a discharge power at the reactive sputtering during at least a part of the period; and forming films on substrates in the sputtering chamber by the reactive sputtering while introducing the substrates to be treated into the sputtering chamber and removing the substrates from the sputtering chamber, one at a time, sequentially.
This embodiment prevents significant excitation of particles sputtered from a sputtering target. This embodiment has an effect, especially on solving the second problem.
The present invention provides a great advantage when the thin film is a metal oxide (especially, indium tin oxide, indium oxide or tin oxide). Furthermore, the present invention is particularly advantageous when the thin film is a transparent conductive film such as an ITO film.
As described above, according to the present invention, in successive formation of thin films on a plurality of substrates by reactive sputtering, the pressure of sputtering gas and the pressure of an intermediate product produced during idling discharge are stabilized by performing idling discharge after the start of the reactive sputtering. Thus, the present invention achieves a method for forming a thin film that can provide all the substrates from the first substrate to the last substrate with constant film characteristics, and a method for forming uniform thin films regardless of the order of the film formation. Therefore, the present invention can achieve good thin film characteristics for a transparent conductive film used in a liquid crystal panel or the like.
Furthermore, according to the present invention, even if thin films are formed on a plurality of substrates, thin films having good characteristics and quality can be formed, regardless of the order of the film formation, by setting a discharge power at idling discharge after the start of the reactive sputtering to a value equal to or lower than at the reactive sputtering.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.